Late hops - IBUs

[tomU]

Just a quick question - how do you work out the IBUs for late hop additions?

In the past I've only worked out IBUs for the bittering additions, but when I add a lot of flavour/aroma hops I do end up with a bitterer beer than I'd want. Presumably hops boiled for 15mins don't just give 1/4 the bitterness of hops boiled for 60mins - that would be far too simple...

[tomU]

apologies, just discovered this:

http://www.realbeer.com/hops/research.html

in another recent post. Thanks Daab